CHAPTER VII 

 INFECTION (Continued) 



PRODUCTION OF DISEASE 



WHEN pathogenic microorganisms have reached the deeper tissues and 

 multiplied, infection has occurred, but, as previously stated, tissue 

 changes of sufficient extent to produce definite lesions and symptoms 

 of disease may or may not result, depending upon whether or not the 

 defensive forces of the host are able to overcome the invaders or are 

 overcome by them. If the latter has occurred, and the invading bac- 

 terium is firmly established in its host, the question of how the bacterium 

 and its products cause disease, that is, the mechanism of the production 

 of an infectious disease, arises for consideration. 



The subject is, indeed, quite complex. Although the etiologic 

 relationship of a large number of pathogenic bacteria to definite patho- 

 logic changes and conditions has been proved by the regularity with 

 which they are found in the diseased tissues, and in many instances has 

 been corroborated by animal experimentation, yet the ways and means 

 by which these bacteria produce disease are quite varied, and are seldom 

 dependent upon one product of bacterial activity. For example, 

 diphtheria and tetanus are apparently simple infections, being caused 

 by soluble toxins secreted by the respective bacilli. There are many 

 factors concerning the action of these toxins, however, which are not as 

 yet understood. Again, the lesions of staphylococcus and other pyo- 

 genic infections are probably due to the activities of soluble toxins, 

 endotoxins, and the protein of the bacterial bodies. All three of these 

 factors are probably concerned in the production of typhoid fever and 

 cholera, whereas the symptoms of sleeping sickness are due in part to 

 blocking of a small but physiologically important vessel in the brain by 

 trypanosomes, with the absorption, at the same time, of toxins and dis- 

 integration products. To these may be added the effects of other 

 biologic activities of the bacteria in living or dead tissues, such as the 

 production of gas from carbohydrates, proteins, etc., in Bacillus aero- 

 genes capsulatus infection. Each infection, therefore, must be regarded 

 as largely a law unto itself, so that all that will be included within the 

 scope of this book will be the mention and illustration of what are 



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